All of today's known PSAs are distinguished by a more or less pronounced flow behaviour. When it is strongly pronounced, this flow behaviour is also known as cold flow or as the bleeding of a PSA. The inherent behaviour of a PSA leads to problems during processing and storage of roll product, and also for its use for diecut materials. For instance, the matrix stripping behaviour of self-adhesive diecut material can be critically disrupted by the flow behaviour of the PSA, leading to an increased reject rate. In order to counteract this it is often necessary to process the material under special temperature conditions; the maximum machine output may be limited, or else the error rate (proportion of remaining diecuts requiring matrix removal) goes up. Similarly, the storage times for the diecut materials are limited and/or special storage conditions are required, generally involving cost and/or inconvenience (for example, the air-conditioning of the storage spaces).
When materials are diecut, they can be kiss-cut or cut right through. Conventional PSAs have considerable disadvantages for use for diecut materials, especially when applied as a layer on a carrier material, in respect of both procedures:                The through-cut materials can be separated from one another only a short time after the diecutting operation. The PSAs coalesce again after the diecutting operation. The coalescence of the PSA occurs in diverse product forms: not only adhesive transfer tapes (PSA applied to a release material) and double-coated materials (PSA on both sides of a carrier, e.g. on foil, paper, web, lay or foam) but also single-coated materials (PSA on one side of a carrier, such as foil, paper, web, lay or foam) show the effect of PSA coalescence after the diecutting operation. The diecuts can no longer be separated from one another without disruption.        Particularly in the case of kiss-cutting, PSA coalescence is accompanied by the following problem: during the kiss-cutting of self-adhesive materials, the release material is cut as well, i.e. the diecutting blades penetrate down to a more or less defined depth into the substrate material (i.e. release material). This is inevitable from a technical standpoint, since in spite of tolerances in the face materials and also operational fluctuations the infeed motion of the die must still ensure reliable severing of the adhesive layer. Consequently the antiadhesively treated surface of the release material is always destroyed (in the majority of cases the release materials are siliconized, but this applies to all release systems described; see Satas, 3rd edition, chapters 26 and 27). The adhesive may flow into the substrate material of the release material (paper, PET, PP, PE) and adhere there. The diecut can no longer be removed unproblematically from the siliconized release material, since the edges of the diecut are stuck to the substrate. In a subsequent processing step the diecut, or the matrix around the diecuts, which is to be removed, may tear during removal. These tears lead to massive disruptions to production, since kiss-cutting operations are carried out primarily in a continuous rotary diecut process.        
The effects described apply to all kinds of product constructions, such as adhesive transfer tapes and also to single-coated and double-coated substrates such as foils, webs, papers, lays or foams.
One approach towards improving the diecutting characteristics of PSA layers is the use of anisotropic, i.e. oriented, PSAs, as are described in DE 100 34 069 A1 or DE 100 52 955 A1.
The orientation of the macromolecules in the PSA plays an important part for other PSA properties as well. During the preparation, further processing or subsequent (mechanical) stressing of polymers or polymer compositions there may be high degrees of orientation of the macromolecules in preferential directions within the polymer assembly as a whole.
The orientation may lead to particular properties in the corresponding polymers. Some examples of properties which can be influenced by the degree of orientation include strength or stiffness of the polymers and/or of the films produced from them, thermal conductivity, thermal stability, and also anisotropy in respect of permeability to gases and liquids. In addition, however, oriented PSAs may exhibit anisotropic stress/strain characteristics and hence also anisotropic adhesive properties, such as under peeling or shearing stress. One important property dependant on the orientation of the units is the refraction of light (expressed by the corresponding refractive index, n) or the retardation, delta. This can therefore be used to determine the orientation. Another method of determining the orientation, particularly in PSAs, is the shrinkback of the free film.
Retention of orientation in partly crystalline rubber PSAs has already been described in U.S. Pat. No. 5,866,249 A. By virtue of the anisotropic adhesive properties it was possible to define innovative PSA applications. There, however, a description is given only of natural rubber PSAs, whose partial crystallization as a result of the orientation is, moreover, a prerequisite for the effects described, and which possess the known disadvantages as compared with acrylate PSAs for example, particularly in respect of ageing stability. The oriented PSA layers are produced in a solventless operation in the course of extrusion coating. This coating technology, however, requires a sharp reduction in the high molecular weight of natural rubber, with the consequence that, in particular, the cohesive properties of the resulting PSAs suffer and necessitate additional crosslinking operations.
DE 100 34 069 A1, DE 100 52 955 A1 and DE 101 57 154 A1 describe processes for preparing oriented acrylate PSAs and are based in each case on the introduction of orientations by means of the coating method—in particular by extrusion coating of the solvent-free PSA and subsequent fixing of the introduced orientations by means of electron-beam or UV irradiation. These methods cannot be applied to solvent-borne PSAs, since in the course of the coating operation any orientations introduced relax immediately because of the high molecular mobility.
Preference for a solventless coating method also poses disadvantages for technical implementation. For instance, owing to the low viscosities required for operation, it is impossible to process PSAs of very high molecular mass and hence great cohesion. In order to achieve high shear strengths it is necessary therefore to crosslink solventlessly coated PSAs after coating, preferably using radiation crosslinking.
Radiation crosslinking has further disadvantages. For instance, electron beams penetrate not only the PSA but also the carrier material, and hence lead to damage to the PSA tape. UV crosslinking is limited in its depth of penetration and restricted to substantially transparent PSAs. With radiation crosslinking, the quality of crosslinking is usually likewise only limited as compared with other crosslinking mechanisms. Furthermore, the cost and complexity of apparatus, particularly for electron-beam irradiation, is high.
EP 0 622 127 A1 describes a process for producing thin PSA films using melt mixing, the intention being to reduce the thickness of the film. The stretching of the film serves solely to reduce thickness.
It is an object of the invention to provide a process for producing highly anisotropic oriented pressure-sensitive adhesives which can be employed universally for all coating methods and which in particular allows the preparation of anisotropic PSAs from solution, dispersion or emulsion. The PSAs ought to possess a sufficient lifetime of the anisotropic state and by virtue of high anisotropy ought to possess favourable properties in respect of their use as PSAs, particularly in respect of their adhesive properties and in the production of diecut products, by avoiding or at least considerably reducing the outlined disadvantages of the prior art.